Multi-gigabit per second (Gbps) communication between various chips or “ports” on a circuit board or modules on a backplane has been in use for quite a while. Data transmission is usually from a transmitter that serializes parallel data for transmission over a communication media, such as twisted pair conductors as a cable or embedded in a backplane, fiber optic cable, or coaxial cable(s), to a receiver that recovers the transmitted data and deserializes the data into parallel form. However, data transmission greater than 8 Gbps over communication paths has been difficult to achieve because various signal impairments, such as intersymbol interference (ISI), crosstalk, echo, and other noise, can corrupt the received data signal to such an extent that a receiver is unable to recover the transmitted data at the desired high data rate with an acceptable level of error performance.
Various techniques are employed to improve the performance of the receiver. One technique is to provide the receiver with a variable gain amplifier (VGA) to assure signal linearity within a desired dynamic range, and a multi-band adjustable analog (linear) equalizer to compensate for frequency-dependent losses. Together these amplifiers might be considered part of the analog front-end equalization (AFE). An adjustable decision feedback equalizer might also be included to compensate for interference cancellation and other non-linear distortions of the channel. Quality of the received signal might be characterized by a shape of the data eye (e.g., the amount of “eye opening”) observed at the receiver.
Even though the quality of the received signal can be improved by the AFE, the complexity of the AFE needed to handle different serial communication protocols (e.g., PCIe Gen3, 12G SAS, SAS-3, 16GFC, and 10GBASE-KR, all of which standard specifications are included herein by reference in their entirety) over communication channels ranging from short, highly reflective channels to long-span channels with a poor insertion loss-to-crosstalk ratio (ICR) may be too complicated to implement cost effectively. Further, the amount of frequency-dependent distortion and interference may exceed the capability of the AFE such that it cannot fully correct for them, resulting in unacceptably poor performance.
One way to improve the quality of the received signal is for the signal transmitter, located in a port coupled to the port with the receiver, to drive the channel with signals that have been pre-distorted by a filter. One such filter used to pre-distort the transmitted signal is a finite-impulse response (FIR) filter with adjustable coefficients or taps, referred to herein as a TXFIR filter. For lower speed applications, the filter coefficients might be predetermined, i.e., selected from a set of preset coefficients, based on the design of the channel and the protocol being implemented. However, with the need for high-speed (e.g., 8 Gbps and above) applications, using a fixed set of coefficients has not worked well for all transmitter/channel/receiver implementations. Even similar implementations may require significantly different TXFIR coefficient values for proper operation due to chip-to-chip electrical parameter variations of the integrated circuits embodying the transmitter and the receiver and the electrical characteristics of the channel media as well.
The standards bodies that administer the various serial communication protocols mentioned above recognized the shortcomings of using fixed TXFIR coefficients and provided in the protocols a feedback mechanism utilizing a back-channel to allow for adjustment of the TXFIR coefficients during initialization of the transmitter and receiver. The protocols allow for the receiver to adapt the TXFIR coefficients by receiving a known data pattern from the transmitter and communicating new coefficient values to the transmitter via the back-channel until a time limit has been reached. Once the time limit has expired, the receiver determines if one or more performance metrics have been met, e.g., eye opening, bit error rate, jitter characteristics, etc. If the performance metrics are not met, then the receiver forces the transmitter to fall back or “down shift” to a slower speed protocol that might also require TXFIR adaptation. If the criteria are met, then the receiver begins other time-consuming initialization processes or begins normal operation. Unfortunately, the time limit for the various protocols can be unnecessarily long (e.g., 24 milliseconds (ms) for PCIe Gen3 and 500 ms for 10GBASE-KR) if successful adaptation occurs well before the time limit expires. The unnecessary adaptation time can cause significant, undesirable delay before normal communication operation begins, particularly when multiple adaptation attempts are made.
Therefore, designers attempt to provide a receiver that can quickly adapt the TXFIR coefficients and determine if the coefficient values have converged to values to allow for the receiver to terminate the adaptation process during initialization before the protocol-specified time period expires, thereby shortening the initialization period of the transmitter and the receiver.
In general, circuit gain dynamic range drops as temperature increases. The receiver is typically adapted at much lower temperature at system start up time, but the operating temperature of the system is generally much greater than the temperature at which the adaptation was performed. The receiver circuit variation over process/voltage/temperature (PVT) results in receiver signal gain variation. The situation for the case when the receiver VGA reaches its maximum value is similar for when the receiver VGA reaches its minimum value. If, at initial adaptation time, the full range of the VGA is utilized then the receiver's circuitry does not have any additional range left to support gain update at steady state operation. As an example, initially the receiver is adapted at sub-100 degree centigrade, and fully utilizes its entire AGC range. At a later time when the circuit heats up to 125 degree centigrade and the received signal is attenuated below desired target operating amplitude range, then the receiver has no room to enhance its operating performance under such PVT condition. Also, at higher operating VGA gain the receiver's effective Nyquist gain is reduced, causing a reduction in effective equalization capability of the receiver.